In this paper, a new kind of symmetry and its conserved quantities of a mechanical system in the phase space are studied. The definition of this new symmetry, i.e., the Lie-Mei symmetry is presented, and the criterion of this new symmetry is also given. The generalized Hojman and the Mei conserved quantities of the Lie-Mei symmetry of the system are obtained. An example is given to illustrative the application of the result.